Semiconductor device

A semiconductor device comprising:

• a gate electrode;
• a gate insulating film over the gate electrode;
• a stacked-layer oxide film over the gate insulating film, the stacked-layer oxide film including a channel formation region;
• and
• a source electrode and a drain electrode each over and in contact with the stacked-layer oxide film,
• wherein:
• the stacked-layer oxide film comprises a first layer, a second layer over the first layer, and a depression portion between the source electrode and the drain electrode,
• the first layer and the second layer each contains In, Ga, Zn, and oxygen,
• the second layer comprises:
• a first region in contact with the source electrode;
• and
• a second region in contact with the drain electrode,
• each of the first region and the second region is in contact with part of side surfaces of the depression portion,
• the first region contains a metal element included in the source electrode,
• the second region contains a metal element included in the drain electrode, and
• the first region and the second region each have a lower resistivity than the channel formation region and comprise an n+ region by diffusion of the metal elements included in the source electrode and the drain electrode, respectively.